I'm using python version 2.4.3

Apparently, when Excel 2003 reads a file, it looks for
the identifying string "ID" at the beginning of the
file. If it finds this string, it assumes it's reading
an SYLK file (see
http://netghost.narod.ru/gff/graphics/summary/micsylk.htm
for some information on SYLK).

The csv module will generate a file that starts with ID
if the first field it writes starts with ID and does
not need to be quoted. When Excel tries to open the
file, the following message pops up:

"Excel has detected that 'test.csv' is a SYLK file, but
cannot load it. Either the file has errors or it is not
a SYLK file format. Click OK to try to open the file in
a different format."

Excel can read the file after clicking OK. Excel
actually has the same problem with CSV files it has
written.

Even so, when using the 'excel' dialect, csv should
write files that Excel can open without any problems.
It could do this by quoting the first field in the file
if it begins with "ID". Unfortunately, csv's Dialect
class does not make this possible. I'm currently
working around it by using QUOTE_NONNUMERIC.